1. Field of the Invention
The present invention relates to a return-less type high-pressure fuel supplying apparatus and method for an internal combustion engine that has a high-pressure pump for supplying high-pressure fuel to the internal combustion engine but does not have a return passage for returning surplus fuel from the high-pressure pump to a fuel tank.
2. Description of Related Art
In a fuel supplying apparatus applied to a generally termed in-cylinder fuel injection type internal combustion engine in which fuel is directly injected into the cylinders of the internal combustion engine, fuel is first pumped from a fuel tank into a fuel gallery via a feed pump, and then introduced into a pressurizing chamber of a high-pressure pump during descent of a plunger of the high-pressure pump. Fuel introduced into the pressurizing chamber is pressurized by the plunger to a pressure that is suitable for in-cylinder injection. Fuel pressurized in this manner then is supplied to a delivery pipe to which fuel injection valves are connected.
In this type of fuel supplying apparatus, after a certain amount of fuel is supplied to the high-pressure pump, a surplus amount thereof, which does not need to be pumped toward the engine, is normally returned to the fuel tank through a return passage. In a typical construction, however, the high-pressure pump is disposed near the internal combustion engine, so that fuel introduced into the high-pressure pump is likely to undergo a temperature increase due to heat from the engine. Therefore, when surplus fuel returns to the fuel tank through the return passage, the fuel temperature in the fuel tank rises, so that the amount of fuel vapor occurring in the fuel tank increases.
As a countermeasure, a return-less type high-pressure fuel supplying apparatus has been proposed (for example, in Japanese Laid-Open Patent Application No. HEI 9-25860) in which the need for a return passage is eliminated by controlling the amount of fuel pumped from the feed pump into the high-pressure pump to a minimum amount that is actually needed. Therefore, this return-less apparatus prevents an undesired event in which heated fuel returns to the fuel tank. Hence, the apparatus is able to prevent or curb fuel temperature increases in the fuel tank and reduce the occurrence of fuel vapor in the fuel tank.
However, since fuel circulation between the high-pressure pump and the fuel tank is eliminated, the return-less type fuel supplying apparatus tends to have relatively high fuel temperatures in the fuel gallery and in the pressurizing chamber. Therefore, if the engine is stopped immediately after the engine temperature has increased due to, for example, a high-load operation or the like, heat from the engine may increase the fuel temperature in the fuel gallery or in the pressurizing chamber approximately to the boiling point of the fuel, so that a portion of the fuel may vaporize and add fuel vapor.
Furthermore, if the engine is restarted at an increased fuel temperature, there is a further increased likelihood of a phenomenon (cavitation) in which when fuel is moved from the fuel gallery into the pressurizing chamber by reciprocations of the plunger, the fuel pressure temporarily drops so that vapor occurs.
The produced vapor is continually moved back and forth between the fuel gallery and the pressurizing chamber by reciprocations of the plunger. If fuel pressurization is performed in the pressurizing chamber when a large amount of vapor has moved into the chamber, the plunger merely compresses fuel vapor, and it becomes difficult for the plunger to sufficiently pressurize liquid fuel. In this manner, the conventional return-less type high-pressure fuel supplying apparatus may fail to secure a predetermined fuel injection pressure when the engine temperature is high. Therefore, the conventional return-less apparatus may cause deterioration of the restarting performance, resulting in, for example, a prolonged time required for a start, occurrence of an engine stall immediately following a start, or the like.